High power ferrite isolator having ferrite materials of differing curie temperature



Jan. 22, 1963 E. N. SKOMAL HIGH POWER FERRITE ISOLATOR HAVING FERRITE MATERIALS OF DIFFERING CURIE TEMPERATURE Filed July 26, 1957 TO LOAD isolation in 0a [FIG.3

O- lmw. input A- 660wutts cw. input 0- 860 watts cw. lnput Insertion Loss in DB BY W |.O

Frequ TO POWER SOURCE EDWARU N. SKOMAL in KM ATTORNEY.

HIGH liQWER FERRITE ISOLATOR HAVING FERRETE MATERIALS (3F DIFFERING CURE TEMPERATURE Edward N. Skornal, Sunnyvale, Calif., assignor, by mesne assignments, to Sylvania Electric Products Inc, Wilmington, Del, a corporation of Delaware Filed duly 26, 15957, Ser. No. 674,373 3 Claims. (Cl. 333-242) This invention relates to microwave components of the type wherein one or more specimens of ferrite or similar material are mounted within a waveguide and are biased with a magnetic field to provide preferential attenuation or isolation, and more particularly to an isolator of this class adaptable for use in hi h average power operation over a broad frequency band.

Basically, a load isolator utilizes one or more ferrite specimens mounted in a wave guide to achieve one-way transmission of microwave energy. It is now well known that when a ferrite element of predetermined size and geometry is biased by a static magnetic field of predetermined strength the waveguide may be made to present a relatively low loss path in one direction and a relatively high loss path in the other direction. Devices of this type have been found to have particular utility in radar systems for non-reciprocally isolating the magnetron or microwave generator from the remainder of the circuit. For example, if the isolator is coupled to the magnetron and oriented such that its forward direction is from the magnetron to the associated duplexer, then substantially all of the power delivered by the magnetron is transmitted to the duplexe-r. However, reflected energy resulting from mismatches and the like which would normally be returned to the magnetron are attenuated or absorbed by the isolator, decreasing the magnetrons sensitivity to load variations. Further, the use of an isolator permits tighter coupling between the magnetron and the load whereby the maximum power output from a system may be increased even though the magnetron output signal is attenuated slightly due to the insertion loss of the isolator.

Although isolators of this kind have enjoyed widespread acceptance, particularly in applications where low average power of relatively constant frequency need be handled, they have been found to have a number of inherent disadvantages which discourage their incorporation in high power systems where broadband performance characteristics must be maintained. lts power handling capacity is limited owing to the fact that the physical disposition of the ferrite in the waveguide prevents rapid dissipation of the heat generated in the ferrite by the absorption of electrical energy; consequently, blowers or a water jacket on the outer surface of the wave guide wall must be employed with the unit if even a moderate amount of power is to be dissipated therein. Also, the static magnetic field strength for a particular degree of isolation and insert-ion loss is critically dependent upon frequencies and the temperature of the ferrite. It has been sought to overcome these deficiencies by employing high Curie temperature ferrites, in this specimens, but even a combination of these techniques, with utilization of cooling as earlier suggested, has failed to provide very high average power load isolator performance characteristics throughout a broad frequency range.

The present invention, on the other hand, provides an improved load isolator which obviates the above and other disadvantages of the load isolator-s of the prior art. According to the basic concept of the invention, one or more ferrite specimens are disposed on one of the broad walls of the wave guide to promote dissipation of heat by conduct-ion through the wall, and the ferrite specimen 3,75,l5 Patented Jan. 22, 196,3

is of multiple composition, a portion having a high Curie temperature and a portion having a somewhat lower Curie temperature and possessing a narrow ferromagnetic resonant line width. The ferrite specimens are in the form of strips disposed longitudinally of the guide, and are disposed such that reflected power first encounters the portion formed of the higher Curie temperature ferrite. The reflected power is principally absorbed in [this portion, but since it is a high Curie temperature material, increases in temperature do not as seriously affect its intrinsic characteristics as if it were a lower Curie temperature ferrite. Thus, this portion protects the lower Curie temperature portion, and the latter is operative to absorb the reflected power not absorbed by the first portion without experiencing appreciable temperature rise whereby its saturization magnetization, and consequently its isolation capabilities, are maintained.

Another feature of theinven-tion is the provision of means for concentrating the energy of the electromagnetic wave propagated through the isolator into the region of the waveguide occupied by the ferrite specimens, thereby to minimize the thickness of the ferrite elements While achieving a high degree of isolation. For an understanding of the significance of this feature it will be advantageous briefly to describe the phenomenon which provides the basis for load isolators of this class. Assuming a rectangular wave guide, with propagation in the dominant or TE mode, the electric component of the electromagnetic wave is a linearly polarized field which varies in amplitude but not in direction and extends across the short dimension of the guide. The magnetic component of the wave loops around the electric field lines in a plane normal to the electric component. To achieve maximum isolation in a ferrite specimen, it is desirable that the magnetic component rotate in a circle within the element; to this end, it is necessary that the magnetic component in the region of the ferrite be rotating elliptically with a predetermined eccentricity. For a given frequency of excitation, there is a unique position across the larger dimension of the wave guide where the magnetic component rotates in an ellipse with exactly the proper eccen tricity to generate a circularly rotating magnetic field in a given ferrite specimen. Thus, if a ferrite specimen is placed in this unique position, good isolation characteristics are obtained. However, as the freq ency of excitation changes, the position where the magnetic component rotates with the proper eccentricity to afford good isolation also changes. While the ferrite specimen might be made large enough to insure that a portion thereof were always in a region of proper eccentricity, this approach has the disadvantage that there would also be a large portion in a region of improper eccentricity which would reduce the ratio of backward to forward attenuation. It is therefore desirable to employ a ferrite specimen which is narrow in the direction of the long dimension of the wave guide, and since it is impractical to adjust the posi-.

fills the guide. By thus concentrating the wave energy, the

rotating magnetic component of the wave in a predetermined portion of that region is caused to have a substantially constant eccentricity of elliptical rotation, even with changes in frequency. By utilizing a piece of dielectric material with suitable dimensions and dielectric constant, elliptical rotation of the magnetic component of the elecv tromagnetic wave, having proper eccentricity to produce circular rotation of the magnetic component within a ferrite, is produced in a region immediately adjacent the dielectric material when the dielectric is positioned on one of the broad walls of the wave guide and on the central longitudinal axis thereof. The dielectric slab is preferably of a thickness so as to extend only partially across the shorter dimension of the guide thereby causing generation of higher order RF modes in the vicinity of the slab. Such higher order modes are necessarily present to a permit propagation of energy through the guide and have maximum amplitude near the vertical edges of the slab. Two thin strips of ferrite material of the multiple composition described earlier are positioned on the same broad wall, adjacent to and on either side of the piece of dielectric where the higher order modes are of maximum amplitude. A pair of elongated permanent magnets positioned exteriorly of the guide, on opposite broad walls, with their pole pieces aligned with the ferrite strips and with opposing poles of opposite polarity, provide skewed static magnetic fields of opposite polarity in the two strips. The concentration of the wave energy, the presence of higher order RF modes and the skewed magnetic fields together provide enhanced non-reciprocal absorption.

It is therefore an object of the invention to provide a load isolator capable of operation at high average power.

Another object of the invention is to provide a load isolator utilizing magnetically biased ferrite specimens having improved characteristics over a broad range of frequencies at high average power.

Another object of the invention is to provide a microwave isolator wherein the ferrite specimens are positioned so as to dissipate heat generated therein by absorption of electrical energy.

The nature of the invention, its application, and further objects and features of novelty will be better appreciated from the following detailed description of a preferred embodiment when considered with the accompanying drawings, in which:

FIG. 1 is an isometric view of one form of load isolator, according to the invention;

FIG. 2 is an isometric view, partially-cut-away, of the wave guide section of FIG. 1 with the flanges and magnets removed; and

FIG. 3 is a graph illustrating the isolation characteristics of an isolator of the type shown in FIGS. 1 and 2 in the 2.8 to 3.6 kmc. frequency range. v r 7 Referring now to the drawings, wherein like parts are designated by like reference characters in FIGS. 1 and 2, there is shown a load isolator constructed in accordance with the principles of the invention. Basically, the present load isolator includes a wave guide section 10, which may include a pair of end flanges 12 and 14, a pair of static magnetic field generators such as two permanent magnets 16 and 18, a pair of ferrite specimens (not shown in FIG. 1) which are mounted within the waveguide symmetrically with respect to the center line, and a strip of dielectric material positioned between the ferrite specimens for concentrating the energy propagated by the guide into-the regions of the ferrite elements and for generating higher order RF modes in said region.

Referring now to FIG. 2, the wave guide portion is shown, partially cut-away and. with the flanges and magnets removed, illustrating the shape and relative positions of the ferrite specimens and the slab of dielectric material for. concentrating the energy propagated in the guide. The dielectric material is preferably in the form of a flat slab 20, formed for example of A1 disposed on one of the broad walls of wave guide 10, substantially along the central longitudinal axis thereof. Slab 20 is preferably of rectangular cross-section, its longer dimension being inthe direction of the long dimension of the guide, and as shown, has a thickness less than the shorter dimension of the guide so as to extend only a portion of the distance between the broad walls. The presence of the dielectric material concentrates the wave energy propagated by the guide into the vicinity of the slab, and since it effectively reduces the shorter dimension of the guide along the central portion, higher order RF modes are generated in the vicinity of the slab to permit propagation of energy down the guide. Such higher order modes have maximum amplitude in the vicinity of the vertical sides of dielectric slab 20, which corresponds generally to the region where the wave energy is concentrated. The degree of concentration of the energy, and the generation of higher order modes are determined mainly by the dimensions of the slab and the dielectric constant of the material, and therefore subject to some variation. By way of example, in an isolator designed for frequencies in the range of 2.8 to 3.6 kilomegacycles and utilizing A1 0 as the dielectric material (dielectric constant=9) a member having a width approximately of the wider dimension of the guide and a thickness approximately /3 of the narrow dimension of the guide has been found to be satisfactory.

On either side of dielectric member, and closely adjacent thereto, and in contact with the same broad wall of the wave guide are positioned ferrite specimens of multiple composition. The ferrite specimens are similar in construction, each comprising a portion 22 formed of high Curie temperature material and possessing a narrow ferromagnetic resonant line width. The ferrite specimens are in the form of strips arranged end-to-end along either side of the dielectric member 20, with the higher Curie temperature specimens positioned at the end of the isolator which is to be connected to the load, and the lower Curie temperature specimens positioned at the end which is to be connected to the source of microwave power. Thus, in the arrangement illustrated, the main power transmission is from right to left, the ferrite specimens absorbing very little power in this direction. The energy propagating in this direction first encounters specimens 24 which account for the predominant portion of the insertion loss, but since they have a narrow line width, the insertion loss is a minimum. The power reflected from the load (from left to right in the isolator) is principally absorbed in high Curie temperature elements 22; being of high Curie temperature material, there must be a substantial increase in temperature (due to energy absorption) before the Curie temperature is approached suiticiently closely as to alfect the isolating characteristics of the material. The elements 22, then, protect the lower Curie temperature elements 24 so that the latter need not absorb appreciable amounts of power whereby the temperature of the latter are also kept well below their Curie temperature. There will, of course, be some heating of both sections of the ferrite specimens, but being in contact with the wall of the wave guide 10, the heat may be removed by conduction to the outer surface of the wave guide where it may be removed by natural convection, or if desired, by a blower. The distribution of the heat by the use of composite ferrite specimens in a manner to extend the operable range of each portion, and the removal of heat from the ferrite material, permits operation of the isolator at high average powers while preserving substantially uniform performance over a broad band of frequencies.

A number of ferrite materials are available which have the desired properties for the portions 22 and 24, and it can be expected that others will result from further in vestigation in this area. Element 22, for example, may be constructed of nickel ferrite, having a Curie temperature of 570 C., and element 24 may be formed of magnesium-manganese ferrite, having a Curie temperature of 300 C. and a ferromagnetic resonant line width of 500 oersteds. As indicated in FIG. 2, the ferrite strips are somewhat narrower in the direction along the wide dimension of the guide than the dielectric member, and also thinner, features tending to reduce the insertion loss of the isolator.

As illustrated in FIG. 1, the isolator employs two magnets 16 and 18 disposed on opposite broad walls of guide of a length slightly greater than that of the composite end-to-end ferrite strips. The pole pieces of magnet 16 are aligned with the ferrite strips, and may be built into the upper broad wall of guide 10 for connecting the ferrite strips to its respective pole through a low reluctance path. With this positioning of magnet 16, the magnetic field lines extend through the two ferrite strips in a skewed manner; that is, not normal to the wider surface of the strips, and it will be realized that the magnetizations in the two strips are in opposite directions. Magnet 18 is similarly disposed on the other broad wall of guide 10 with its pole pieces aligned with the pole pieces of magnet 16, except that opposing poles are of opposite polarity. Magnet 18 together with magnet 16 produces a magnetic field of opposite polarity in the ferrite strips, but in view of the longer and higher reluctance path between opposed poles of magnets 16 and 18 than between the poles of magnet 16, the resultant magnetic fields in the ferrite strips is somewhat askew but generally in opposite directions as indicated in FIG. 2 by the arrows marked H Magnets 16 and 18 may be formed of Alnico V or other suitable magnet material, and while a generally semi-circular cross-section is shown, the magnets may be of any shape so long as the pole piece faces are generally normal to the wave guide walls at the lines of contact, and in alignment with the ferrite strips.

In FIG. 3 there is shown a plot of insertion loss and isolation versus frequency at three different power levels for an S-band load isolator of the type shown in FIGS. 1 and 2, and having the following dimensions and characteristics:

Dimensions of wave guide 10 0.43" x 3". Longitudinal length of member 20--. 6". Width of member 20 Thickness of member 20 0.2". Material of member 20 A1 0 Longitudinal length of portions 22--. ,6 Width of portions 22 and 24 A Thickness of portions 22 and 24--..-. .040". Material of portions 22 NiFe O Longitudinal length of portions 24-. 2 Material of portions 24 MnMgFe O Average input powers 1 milliwatt,

660 watts C.W., 360 watts C.W.

From the curves of FIG. 3 it is seen that the present load isolator is capable of providing large ratios of isolation loss to insertion loss when operating in very high power (peak and average) microwave systems, and over a wide frequency range. To summarize briefly, this improvement of capability over prior art isolators is accomplished by the utilization of composite ferrite specimens, a high Curie temperature portion and a narrow line width portion, positioned in contact with a dielectric member for concentrating the wave energy into the region of the ferrite specimens and to generate higher order RF modes in said region, the ferrite specimens being in contact with a wall of the wave guide to promote conduction of heat therefrom. The higher Curie temperature ferrite is positioned to absorb the principal portion of unwanted incident power thereby protecting the lower Curie temperature ferrite against deleterious heating. This configuration of the ferrite strips with t e dielectric memher, magnetized in an askewed manner, provides en hanced non-reciprocal absorption.

While a specific embodiment of an isolator has been shown in the drawings to illustrate the basic concept of the invention, it will be understood that modifications in the invention will be readily apparent to one skilled in the art. For example, each of the ferrite specimens might be formed of three or more portions of descending Curie temperature material in the direction from load to power source. Also, the concept of employing ferrite specimens having portions of unlike characteristics as above-described may be used in isolators of the type having ferrite elements asymmetrically positioned on one or both broad walls of the guide as shown, for example in Sparling Patent 2,776,412; i.e., in isolators which do not employ a dielectric member for concentrating the field. Accordingly, the scope of the invention is to be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. In a microwave component for isolating a microwave source from a load wherein at least one ferrite specimen is mounted entirely within a rectangular wave guide and is subjected to an externally applied magnetic field, a ferrite specimen having two or more end-to-end portions formed of ferrite material of differing Curie temperature with the higher Curie temperature portion positioned at the load end of said wave guide section.

2. An absorption type load isolator for isolating a microwave source from a load comprising, a section of rectangular wave guide, at least one ferrite specimen mounted within said wave guide on a broad wall thereof asymmetrically with respect to the central longitudinal axis of the said wall, said ferrite specimen having at least two end-to-end portions formed of ferrite material of difiering Curie temperature, the portion formed of the higher Curie temperature material being positioned at the load end of said wave guide section, and external means for subjecting said ferrite specimen to a static magnetic field.

3. An absorption type load isolator for isolating a microwave source from a load comprising, a section of rectangular wave guide, a strip of dielectric material mounted within said wave guide on a broad wall thereof along the central longitudinal axis of the said wall, at least one ferrite strip mounted on said wall adjacent said of dielectric material, said ferrite strip having at least two end-to-end portions formed of ferrite material of differing Curie temperature, the portion formed of the higher Curie temperature material being positioned at the load end of said wave guide section, and external means for providing a static magnetic field in said ferrite strip.

References Cited in the file of this patent UNITED STATES PATENTS Sparling Jan. 1, 1957 Sensiper Sept. 17, 1957 OTHER REFERENCES 

1. IN A MICROWAVE COMPONENT FOR ISOLATING A MICROWAVE SOURCE FROM A LOAD WHEREIN AT LEAST ONE FERRITE SPECIMEN IS MOUNTED ENTIRELY WITHIN A RECTANGULAR WAVE GUIDE AND IS SUBJECTED TO AN EXTERNALLY APPLIED MAGNETIC FIELD, A FERRITE SPECIMEN HAVING TWO OR MORE END-TO-END PORTIONS FORMED OF FERRITE MATERIAL OF DIFFERING CURIE TEMPERATURE WITH THE HIGHER CURIE TEMPERATURE PORTION POSITIONED AT THE LOAD END OF SAID WAVE GUIDE SECTION. 